The present invention relates to an output circuit, and more particularly to an output circuit for driving a loudspeaker with an output of a monopolar (i.e. single directional) current output type digital-analog (D/A) converter.
In recent years, with the progress of the digital information processing technique, various electronic instruments adapted to deliver output information directly by means of speech without employing a display or the like such as, for example, a speech or sound output type desk computer, a language exercising machine, etc. have been developed.
In these speech or sound output type electronic instruments it has been a common practice that every information is basically digitally calculated and processed and the resultant digital information is further converted into analog information so that a loudspeaker serving as an output device is driven by the analog information. In the heretofore known speech output type electronic instrument, the digital-analog (D/A) converter is applied with a reference potential (e.g. ground potential) and either a positive or negative power supply potential as an operating power supply, and hence the output of the D/A converter can take only a monopolar value. Accordingly, it is impossible to drive a loudspeaker by making direct use of such a D/A converted output. Therefore, in the prior art electronic instrument, provision is made such that a loudspeaker may be driven by the D/A converted output via an output transformer or an A.C. amplifier (audio amplifier). Consequently, it was difficult to manufacture a speech output type electronic instrument at low cost. In addition, as a matter of course, the use of an output transformer or an A.C. amplifier prevents reduction in size of the instruments. It is also to be noted that, although it is possible to drive a loudspeaker by applying a monopolar D/A converter output directly thereto without employing an output transformer or the like, it is necessary to continuously feed a bias current approximately equal to a medium current value of the D/A converter output current to the loudspeaker so that the loudspeaker may be fed a current swinging in the both the power supply voltage and the reference voltage directions in correspondence to the output current swing relative to the medium current value of the D/A converter current. Consequently, despite a large power consumption the effective output is reduced to only 1/4 of the output power.